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AC electrograining of aluminum plate in hydrochloric acid
Materials Chemistry and Physics 68 (2001) 217–224
AC electrograining of aluminum plate in hydrochloric acid C.S. Lin∗ , C.C. Chang, H.M. Fu Department of Mechanical Engineering, Da-Yeh University, 112, Shan-Jeau Road, Dah-Tsuen, Changhua 51505, Taiwan, ROC Received 18 January 2000; received in revised form 23 May 2000; accepted 8 June 2000
Abstract The evolution of etch pits on the AA1050 aluminum lithographic plate was studied. AC electrograining was carried out in a 35◦ C, 0.16 M hydrochloric acid solution at a current density of 15 Apeak dm−2 and a frequency of 50 Hz. Scanning electron microscopy was used to characterize the surface morphology of electrograined aluminum. An epoxy replica technique was employed to reveal the internal structure of etch pits. Cross-sectional transmission electron microscopy was performed to reveal the detailed pit morphology and the microstructure of etch film. Together with the measurement of surface properties such as Ra , Rmax , peak count and capacitance, a pit growth sequence is described. Three types of pits are observed during the first stage of electrograining: the fine pit, hemispherical pit and worm-like pit. The mergence of fine pits results in hemispherical pit. Lateral and unidirectional growth of hemispherical pits creates worm-like pit. A uniformly pitted surface is achieved when most of the fine pits merge into hemispherical pits. At this stage of electrograining, the worm-like pits disappear as they coalesce with hemispherical pits. TEM observations further illustrate that the hemispherical pits formed up to this stage are shallow in base. With further electrograining, new hemispherical pits form on the walls of existing hemispherical pits, leading to the formation of deeper pits. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrograining; AA1050 aluminum lithographic plate; Hemispherical pit; Worm-like pit
1. Introduction Pre-sensitized aluminum plates are used extensively for lithographic printing, which is based on the operation of the oleophilic image areas which accept and transfer ink during printing process and the hydrophilic non-image areas which accept water or aqueous solution during printing to repel such greasy inks [1–10]. A uniformly roughened surface is an important property for the aluminum lithographic plate in enhancing the adhesion of ink and the retention of water. Several graining methods have been adapted to roughen the surface of aluminum plates including mechanical, chemical and electrolytic graining [1]. The aluminum plates exhibit a characteristic pitted and convoluted surface after electrograining. The morphology and size distribution of etch pits depends on the electrolyte composition, the graining conditions and the properties of aluminum plate [1–14]. Thompson and Wood [15] investigated the pitting behavior of aluminum electrodes in hydrochloric acid under an applied alternating voltage. Pit formation was thought to occur by local attack at flaw in the air-formed film on the aluminum substrate. Pitted and convoluted surface topography developed upon merging of larger pits and undermining ∗ Corresponding author. E-mail address: [email protected] (C.S. Lin).
of the surface film. It is generally recognized that the aluminum surface after AC etching is covered with etch film [4–16], which is mainly composed of hydrated aluminum [12,15]. The simultaneous etch film formation and its subsequent incomplete destruction tends to roughen the aluminum surface, a process to form pitted and convoluted surface [4,5]. The film weight of etch product is a function of electrolyte and graining conditions [5]. Surface morphology of the electrograined aluminum plate has been extensively studied by Terryn and co-workers [6,7,11–14]. In the study of AC electrograining of aluminum in hydrochloric and nitric acids, they illustrated that the morphological building elements and their ordering were fundamentally different, although the hemispherical pits were observed for treatment in both electrolytes. The basic elements building up the morphology of surface treated in nitric acid were found to be a small flat walled hemispherical pit, formed upon the random dissolution of aluminum. Conversely, the fine cubic pits were found to be the basic building elements for surface treated in hydrochloric acid. While no characteristic dimension was observed for the hemispherical convoluted pits, both morphological building elements exhibited a characteristic dimension of 1–2 m at an AC frequency of 1 Hz and a current density of 1500 Arms m−2 . The effective size of the building elements increased with increasing current density and decreasing graining frequency.
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In the present work, we report a detailed cross-sectional transmission electron microscopy (TEM) observation on the AA1050 aluminum plates electrograined in hydrochloric acid. Surface morphology of the aluminum plates after sequential electrograining was observed using scanning electron microscopy (SEM). An epoxy replica method was used to reveal the interior structure of etch pit. Together with the measurement of Ra , Rmax , peak count and relative increase factor, the pit growth sequence is discussed.
2. Experimental 2.1. Electrograining of AA1050 aluminum plates Commercial grade 0.25-mm-thick AA1050 aluminum plates, after degreasing in a 50◦ C, 5% NaOH solution, were electrolytically grained in a 0.16 M hydrochloric acid at a temperature of 35◦ C using a flow cell. AC electrograining was performed at a current density of 15 Apeak dm−2 and frequency of 50 Hz, provided by an EG&G373 Potentiostat/Galvanostat together with an EG&G365 current booster. Sinusoidal signal was generated by an HP 8116A pulse function generator. The electrograining time ranged from 30 to 300 s so that pit growth could be monitored. Two aluminum plates were electrograined at each time. One of the electrograined plates was dried at room temperature and was reserved for cross-sectional TEM characterization. The other plate was immersed in a phosphoric and chromic acid solution to remove the etch film. The properties of electrograined surface were then investigated. 2.2. Property evaluation Increase in the effective surface area of aluminum plate during electrograining was semi-quantified via the measurement of the capacitance of aluminum plate. The relative increase factor of effective surface area was defined as the capacitance of electrograined aluminum plate divided by that of as-received aluminum plate. Surface profiles of electrograined aluminum plate were measured using Tokyo Seimitsu Surfcom instrument. Arithmetic mean roughness, Ra , and maximum roughness, Rmax , were recorded. Moreover, peak count, defined as the number of peak and valley on the profile with peak height and/or valley depth larger than 0.2 m, was measured on the 8 mm scanning length. 2.3. Microstructural characterization Surface morphology of the electrograined aluminum plate of which the etch film had been removed was investigated using an SEM. An epoxy replica method was employed to reveal the internal structure of etch pit. A 7 mm×7 mm square of electrograined aluminum plate was placed in a BEEM embedding capsule with the grained side up. The
capsule was subsequently filled with Spurr epoxy. Before cured at 60◦ C for 12 h, the capsule was evacuated within an oven using rotary pump for half an hour to ensure the epoxy completely fill in the whole space of etch pit. After peeling the capsule off, the epoxy-embedded aluminum plate was immersed in a 10% NaOH solution for 4 h to dissolve the aluminum substrate. The epoxy replica then duplicated the surface morphology of the electrograined aluminum plate. Cross-sectional TEM specimens were made from the as-grained aluminum plate using a combined mechanical grinding and ion-beam thinning method. Ion-beam thinning was carried out using Gatan dual-milling machine at a voltage of 5 keV. The time for ion milling was usually less than 2 h. TEM specimens were examined on a conventional JOEL200CXII TEM and a JOEL3010 analytical TEM. Along with detailed microstructure, particularly the pit morphology, energy-dispersive spectroscopy (EDS) was used for the composition analysis of etch film.
3. Results and discussion 3.1. Surface morphology of electrograined aluminum plates Fig. 1 shows a series of SEM micrographs illustrating the surface morphology change of aluminum plates with graining time. The as-rolled aluminum plate exhibits a relatively smooth surface with characteristic rolling lines, which are still discernible after 30 s of electrograining (arrow in Fig. 1(a)). Numerous pits form after 30 s of electrograining. Three types of pit were observed: the fine pit (marked as F in Fig. 1(a) and (b)), hemispherical pit (marked as H in Fig. 1(a) and (b)), and worm-like pit (marked as W in Fig. 1(a) and (b)). Most of the surface is dotted with fine pits. Several relatively large hemispherical and worm-like pits are also noted. The size of fine pits is usually far less than 1 m. The typical size of hemispherical pits is approximately 2 m. The typical length of worm-like pits is about 5 m. The orientation of worm-like pits is random with respect to the rolling direction. As the graining sequence proceeds to 60 s, most of the aluminum surface is dotted with pits. Growth and coalescence of fine pits result in the formation of more hemispherical pits. Meanwhile, the existing relatively large hemispherical pits show little growth. In addition, the population density and size of worm-like pits remains constant as electrograining time increases from 30 to 60 s. The original rolling lines are no longer visible at this stage of electrograining since most of the aluminum surface is dotted with pits. With further electrograining, more hemispherical pits form and the areas dotted with fine pits decrease. Concurrently, the worm-like pits merge with the growing hemispherical pits and presumably disappear (arrow in Fig. 1(c)). After 120 s of electrograining, most of the aluminum surface is dotted with relatively large hemispherical pits (Fig. 1(d)). The remaining fine pits always reside along some of the boundaries between hemispherical pits.
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Fig. 1. Series of SEM micrographs showing surface morphology change with electrograining time of (a) 30, (b) 60, (c) 90, (d) 120, (e) 180, (f) 240 s.
Note that at this stage of electrograining, the size distribution of pits is uniform, since more than 90% of the surface is dotted with hemispherical pits. With further electrograining (Fig. 1(e), 180 s of electrograining), the hemispherical pits grow laterally as they merge with the surrounding hemispherical pits. The average size of hemispherical pits is approximately 5 m. Meanwhile, the hemispherical pits propagate into the aluminum substrate via the formation of new pits on the walls of existing hemispherical pits. Consequently, the depth of hemispherical pits exhibits a more pronounced increase as electrograining time exceeds 180 s. Large and deep hemispherical pits form after 300 s of electrograining (Fig. 1(f)). Laevers et al. [7] investigated the influence of manganese on the AC electrolytic graining of aluminum. It is demonstrated that the degree of uniformity of the final pit morphology increases with decreasing amount of manganese present
in solid solution. For example, a uniformly pitted and convoluted surface is obtained for the 99.99% purity aluminum and the 1 wt.% manganese alloyed aluminum plate with most of manganese precipitated in intermetallic particles. Conversely, for 1 wt.% of manganese alloyed aluminum plate with most of the manganese (0.9 wt.%) present in solid solution, a lateral and unidirectional growth of hemispherical pits occurs, creating worm-like pits. The worm-like pits observed in the present work may arise from the alloying elements present in solid solution, such as iron, silicon and titanium for the AA1050 aluminum plate. The coexistence of hemispherical pits and worm-like pits suggests that the total amount of solid solution elements in the AA1050 aluminum plates is not enough to induce the prevalent formation of worm-like pits since the total amount of alloy elements present in AA1050 aluminum plate is approximately 0.5 wt.%.
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Fig. 2. Series of SEM micrographs showing internal structural change of etch pit with electrograining time of (a) 30, (b) 60, (c) 90, (d) 120, (e) 180, (f) 240 s.
3.2. Internal structure of etch pit Fig. 2 illustrates a series of SEM micrographs showing the internal structure of the electrograined surfaces. Similarly, the fine pit (marked as F in Fig. 2(a) and (b)), hemispherical pit (marked as H in Fig. 2(a) and (b)) and worm-like pit (marked as W in Fig. 2(a)) are the typical features on the aluminum surface after 30 s of electrograining. After 60 s of graining, most of the surface is dotted with etch pits (Fig. 2(b)). The walls of hemispherical pits are also dotted with many fine pits (arrow in Fig. 2(b)). As electrograining proceeds, growth and coalescence of fine pits lead to the formation of more hemispherical pits. Meanwhile, no apparent growth of the existing relatively large hemispherical pits was observed. The disappearance of worm-like pits arises from their merge with the hemispherical pits (arrow
in Fig. 2(c), 90 s of electrograining). Most of the aluminum surface is dotted with hemispherical pits after 120 s of graining (Fig. 2(d)). Growth and coalescence of fine pits on the walls of hemispherical pit form new hemispherical pits (arrow in Fig. 2(d)) inside the existing hemispherical pit. This is the typical process that the existing hemispherical pit propagates into the aluminum substrate. Meanwhile, lateral coalescence of hemispherical pits occurs progressively as electrograining proceeds, leading to the formation of the larger and deeper hemispherical pits (Fig. 2(e) and (f)). 3.3. Cross-sectional TEM microstructural characterization Cross-sectional TEM reveals more detailed morphology of various pits. Fig. 3 shows the morphology of a worm-like pit that is generally recognized by its longer length as com-
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Fig. 3. Cross-sectional TEM micrograph showing the microstructure of a worm-like pit (electrograined for 60 s).
Fig. 4. Cross-sectional TEM micrograph showing the microstructure of a hemispherical pit (electrograined for 90 s).
pared to the hemispherical pit and fine pit. For example, the typical length of the worm-like pits is approximately 5 m after 60 s of electrograining. Conversely, the size of relatively large hemispherical pits is approximately 2 m. It is evident that the worm-like pit is rather shallow in base, i.e. the pit is wider than its depth. The surface of worm-like pit is dotted with fine pits (arrow in Fig. 3), which are also
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Fig. 5. Cross-sectional TEM micrograph showing the microstructure of area dotted with fine pit and hemispherical pit (electrograined for 90 s).
hemispherical when resolved using TEM. The large pit on the samples electrograined more than 90 s is characterized as the hemispherical pit since the worm-like pits formed during the early stage of electrograining presumably disappear after 90 s of electrograining. Fig. 4 illustrates such an example. It is noted that the shape of hemispherical pit observed by SEM is not exactly hemispherical as shown by cross-sectional TEM. Besides, instead of curved surface, flat wall is observed associated with the hemispherical pit. Fig. 5 shows the morphology of the area dotted with fine pit (arrow) and hemispherical pit (double arrows) which also has a shallow base. Although the fine pits are close to hemisphere, their depths are smaller than those of hemispherical pits. Fig. 6 shows a large hemispherical pit on the aluminum surface after 120 s of electrograining. Again, the base of hemispherical pit is quite shallow, indicating that the lateral growth of hemispherical pit prevails its propagation into the aluminum substrate. Moreover, the hemispherical pit is across several subgrains. The structure of subgrain shows little effect on the morphology of the hemispherical pit. Conversely, the subgrain boundary has been identified as the pit nucleation site during the beginning of AC electrograining [9]. Electrochemical etching of aluminum in hydrochloric acid results in cubic pits [16,17]. Terryn and co-workers [6,7,11,13,14] concluded that the cubic pits are the basic
Fig. 6. Cross-sectional TEM micrograph showing the microstructure of a large hemispherical pit (electrograined for 120 s).
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Fig. 7. Cross-sectional TEM micrograph showing the etch pit structure of the AA1050 aluminum electrograined at 10 Hz.
building elements for pitted and convoluted surface of aluminum electrograined in hydrochloric acid. Characteristic size of the cubic pits decreases with increasing frequency of AC current. For electrograining frequency of 1 and 10 Hz, the characteristic size of cubic pit is about 1–2 and 0.3 m, respectively. The fine pits observed in the present work using cross-sectional TEM have a hemispherical shape. We also observed the cubic pits (arrow in Fig. 7) of characteristic size of 0.1 m decorated on the surface of the AA1050 aluminum plate, which is electrograined with the same graining conditions used for this study, except that 10 Hz electrograining frequency is adopted. Evidently, the size of basic building element is quite small when 50 Hz frequency is used, since the walls of fine pits are rather smooth. Terryn et al. [6,11] confirmed that the walls of hemispherical pits become smooth as the size of cubic pits decreases with increasing AC frequency. The formation of cubic pits indicates that the attack of aluminum in hydrochloric acid is along preferred crystallographic directions. The walls of hemispherical pits are flat instead of curved, suggesting that the pits develop along preferred crystallographic directions although the fine cubic pits cannot be resolved. The pits tend to grow laterally instead of propagation into the aluminum substrate since the base of hemispherical pit is shallow and flat. This is consistent with the observations made by Terryn and co-workers [6,7,11,13,14]. During AC etching, etch products always form to cover on top of grained surface. Fig. 8 shows a thin etch film on top of the electrograined surface. The etch film contains many fine pores (arrow in Fig. 8(a)), implying that the etch film is a porous structure. EDS reveals that the etch film is composed of aluminum and oxygen. This is consistent with the previ-
Fig. 8. (a) Cross-section TEM micrograph showing the microstructure of etch film on the as-grained aluminum surface and (b) the selected area diffraction pattern.
ous results illustrating that etch film is mainly composed of aluminum hydroxide [12,15]. Selected area diffraction pattern exhibits amorphous halo from the etch film, indicating that the etch film is amorphous (Fig. 8(b)). The diffraction spots (arrow in Fig. 8(b)) are from aluminum substrate. It is also noted that the amorphous etch film tends to crystallize when irradiated by 200 keV electrons. Therefore, instead of diffuse amorphous halo, the selected area diffraction pattern recorded shows faint amorphous halo. 3.4. Surface properties of electrograined aluminum plate Fig. 9 illustrates the dependence of Ra and Rmax on electrograining time. Ra and Rmax of electrograined aluminum plates increase progressively with electrograining time up to 300 s of graining. The lateral size of hemispherical pits as measured on SEM micrographs is larger than the corresponding Ra . This is consistent with the shallow-based hemispherical pits observed using cross-sectional TEM. The relative increase factor of aluminum plates increases abruptly during the first 30 s of electrograining treatment, which tends to roughen the aluminum surface (Fig. 10). The relative increase factor reaches a maximum after 90 s of electrograining, followed by a decrease after 120 s of electrograining, when most of the aluminum surface is dotted with the hemispherical pits. Evidently, the decrease in effective surface area arises from the disappearance of fine pits on the original aluminum surface. An increase in the relative increase factor is, however, noted when graining time increases from
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Fig. 9. Dependence of Ra and Rmax on the electrograining time.
Fig. 10. Dependence of peak count and relative increase factor on the electrograining time.
240 to 300 s. The fine pits decorated on the walls of deeper hemispherical pits are thought to bring about the increase of effective surface area of grained surface. The peak count of grained aluminum surface increases with electrograining time up to 120 s, when most of the fine pits have merged to form hemispherical pits. Peak count, then, decreases with electrograining time as the hemispherical pits grow and coalesce each other. Peak count remains constant as graining time exceeds 180 s, implying that new hemispherical pits formed inside the existing hemispherical pit compensates
for the decrease in peak count due to the coalescence of existing hemispherical pits. 4. Conclusions Pit growth of the AA1050 aluminum plates electrograined in hydrochloric acid was studied, with focus on the evolution of etch pits as electrograining proceeds. The fine pit, hemispherical pit and worm-like pit develop on the aluminum surface after 30 s of electrograining. The hemispherical pits
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form upon the coalescence of fine pits. Lateral and unidirectional growth of hemispherical pits result in the formation of worm-like pits. The population density of hemispherical pits increases with electrograining time. After 120 s of electrograining, a uniformly pitted surface is achieved with most of the aluminum surface dotted with hemispherical pits. Conversely, the population density and size of worm-like pits show little change as electrograining time increases from 30 to 60 s. Coalescence of the worm-like pits with hemispherical pits result in the breakdown of worm-like pits, which is then presumably phased out after 120 s of electrograining. The worm-like pit developed during the early stage of electrograining shows little effect on the uniformity of pit distribution after 120 s of electrograining. With further electrograining, lateral growth and coalescence of hemispherical pits result in the formation of larger hemispherical pit. In addition, deeper hemispherical pits form as fine pits nucleated on the walls of existing hemispherical pits merge each other to form new hemispherical pits inside the existing hemispherical pits. The hemispherical pit is not exactly hemispherical when observed using TEM. The base of hemispherical pit is shallow and its walls are flat. TEM resolves that fine pits also have a shape close to hemisphere. The etch film on top of the grained surface is identified as porous amorphous aluminum hydroxide. The subgrain structure shows little effect on the growth of hemispherical pit. The surface properties of electrograined aluminum plate correlate well with the corresponding pit morphology. Both Ra and Rmax increase as electrograining proceeds. A maximum peak count is achieved when most of the aluminum surface is dotted uniformly with hemispherical pits. The relative increase factor reaches a maximum value right before the stage when most of the fine pits merge into hemispherical pits.
Acknowledgements This research was supported by National Science Council, Republic of China, under grant No. 882216E212001. The authors would like to thank China Steel, Aluminum Corporation, for providing materials for this study. Special thanks to Mr. S.H. Hsieh of China Steel Corporation for his invaluable discussion. Also, we like to thank Mr. I.T. Wang, China Steel Corporation, for his help on carrying out the electrograining experiments.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
A. Nishino, T. Kakei, US Patent 5,141,605 (1992). O. Gobbetti, US Patent 5,064,511 (1991). J.E. Walls, R.I. Dragon, A. Dunder, US Patent 4,336,113 (1982). T. Suzuki, Y. Hayashi, J. Surf. Finish. Jpn. 30 (1979) 541. A.J. Dowell, Trans. Inst. Met. Finish. 57 (1979) 138. H. Terryn, J. Vereecken, G.E. Thompson, Trans. Inst. Met. Finish. 66 (1988) 116. P. Laevers, H. Terryn, J. Vereecken, B. Kernig, B. Grzemba, Corros. Sci. 38 (1996) 413. B. Kernig, B. Grezmba, G. Scharf, Trans. Inst. Met. Finish. 70 (1992) 190. G.J. Marshall, J.A. Ward, Mater. Sci. Technol. 11 (1995) 1015. M.P. Amor, J. Ball, Corros. Sci. 40 (1998) 2155. H. Terryn, J. Vereecken, G.E. Thompson, Corros. Sci. 32 (1991) 1159. H. Terryn, J. Vereecken, G.E. Thompson, Corros. Sci. 32 (1991) 1173. P. Laevers, H. Terryn, J. Vereecken, Trans. Inst. Met. Finish. 70 (1992) 105. P. Laevers, H. Terryn, J. Vereecken, G.E. Thompson, Corros. Sci. 35 (1993) 231. G.E. Thompson, G.C. Wood, Corros. Sci. 18 (1978) 721. C.K. Dyer, R.S. Alwitt, J. Electrochem. Soc. 128 (1981) 300. R.S. Alwitt, H. Ucji, T.R. Beck, R.C. Alkire, J. Electrochem. Soc. 131 (1984) 13.
AC electrograining of aluminum plate in hydrochloric acid C.S. Lin∗ , C.C. Chang, H.M. Fu Department of Mechanical Engineering, Da-Yeh University, 112, Shan-Jeau Road, Dah-Tsuen, Changhua 51505, Taiwan, ROC Received 18 January 2000; received in revised form 23 May 2000; accepted 8 June 2000
Abstract The evolution of etch pits on the AA1050 aluminum lithographic plate was studied. AC electrograining was carried out in a 35◦ C, 0.16 M hydrochloric acid solution at a current density of 15 Apeak dm−2 and a frequency of 50 Hz. Scanning electron microscopy was used to characterize the surface morphology of electrograined aluminum. An epoxy replica technique was employed to reveal the internal structure of etch pits. Cross-sectional transmission electron microscopy was performed to reveal the detailed pit morphology and the microstructure of etch film. Together with the measurement of surface properties such as Ra , Rmax , peak count and capacitance, a pit growth sequence is described. Three types of pits are observed during the first stage of electrograining: the fine pit, hemispherical pit and worm-like pit. The mergence of fine pits results in hemispherical pit. Lateral and unidirectional growth of hemispherical pits creates worm-like pit. A uniformly pitted surface is achieved when most of the fine pits merge into hemispherical pits. At this stage of electrograining, the worm-like pits disappear as they coalesce with hemispherical pits. TEM observations further illustrate that the hemispherical pits formed up to this stage are shallow in base. With further electrograining, new hemispherical pits form on the walls of existing hemispherical pits, leading to the formation of deeper pits. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrograining; AA1050 aluminum lithographic plate; Hemispherical pit; Worm-like pit
1. Introduction Pre-sensitized aluminum plates are used extensively for lithographic printing, which is based on the operation of the oleophilic image areas which accept and transfer ink during printing process and the hydrophilic non-image areas which accept water or aqueous solution during printing to repel such greasy inks [1–10]. A uniformly roughened surface is an important property for the aluminum lithographic plate in enhancing the adhesion of ink and the retention of water. Several graining methods have been adapted to roughen the surface of aluminum plates including mechanical, chemical and electrolytic graining [1]. The aluminum plates exhibit a characteristic pitted and convoluted surface after electrograining. The morphology and size distribution of etch pits depends on the electrolyte composition, the graining conditions and the properties of aluminum plate [1–14]. Thompson and Wood [15] investigated the pitting behavior of aluminum electrodes in hydrochloric acid under an applied alternating voltage. Pit formation was thought to occur by local attack at flaw in the air-formed film on the aluminum substrate. Pitted and convoluted surface topography developed upon merging of larger pits and undermining ∗ Corresponding author. E-mail address: [email protected] (C.S. Lin).
of the surface film. It is generally recognized that the aluminum surface after AC etching is covered with etch film [4–16], which is mainly composed of hydrated aluminum [12,15]. The simultaneous etch film formation and its subsequent incomplete destruction tends to roughen the aluminum surface, a process to form pitted and convoluted surface [4,5]. The film weight of etch product is a function of electrolyte and graining conditions [5]. Surface morphology of the electrograined aluminum plate has been extensively studied by Terryn and co-workers [6,7,11–14]. In the study of AC electrograining of aluminum in hydrochloric and nitric acids, they illustrated that the morphological building elements and their ordering were fundamentally different, although the hemispherical pits were observed for treatment in both electrolytes. The basic elements building up the morphology of surface treated in nitric acid were found to be a small flat walled hemispherical pit, formed upon the random dissolution of aluminum. Conversely, the fine cubic pits were found to be the basic building elements for surface treated in hydrochloric acid. While no characteristic dimension was observed for the hemispherical convoluted pits, both morphological building elements exhibited a characteristic dimension of 1–2 m at an AC frequency of 1 Hz and a current density of 1500 Arms m−2 . The effective size of the building elements increased with increasing current density and decreasing graining frequency.
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In the present work, we report a detailed cross-sectional transmission electron microscopy (TEM) observation on the AA1050 aluminum plates electrograined in hydrochloric acid. Surface morphology of the aluminum plates after sequential electrograining was observed using scanning electron microscopy (SEM). An epoxy replica method was used to reveal the interior structure of etch pit. Together with the measurement of Ra , Rmax , peak count and relative increase factor, the pit growth sequence is discussed.
2. Experimental 2.1. Electrograining of AA1050 aluminum plates Commercial grade 0.25-mm-thick AA1050 aluminum plates, after degreasing in a 50◦ C, 5% NaOH solution, were electrolytically grained in a 0.16 M hydrochloric acid at a temperature of 35◦ C using a flow cell. AC electrograining was performed at a current density of 15 Apeak dm−2 and frequency of 50 Hz, provided by an EG&G373 Potentiostat/Galvanostat together with an EG&G365 current booster. Sinusoidal signal was generated by an HP 8116A pulse function generator. The electrograining time ranged from 30 to 300 s so that pit growth could be monitored. Two aluminum plates were electrograined at each time. One of the electrograined plates was dried at room temperature and was reserved for cross-sectional TEM characterization. The other plate was immersed in a phosphoric and chromic acid solution to remove the etch film. The properties of electrograined surface were then investigated. 2.2. Property evaluation Increase in the effective surface area of aluminum plate during electrograining was semi-quantified via the measurement of the capacitance of aluminum plate. The relative increase factor of effective surface area was defined as the capacitance of electrograined aluminum plate divided by that of as-received aluminum plate. Surface profiles of electrograined aluminum plate were measured using Tokyo Seimitsu Surfcom instrument. Arithmetic mean roughness, Ra , and maximum roughness, Rmax , were recorded. Moreover, peak count, defined as the number of peak and valley on the profile with peak height and/or valley depth larger than 0.2 m, was measured on the 8 mm scanning length. 2.3. Microstructural characterization Surface morphology of the electrograined aluminum plate of which the etch film had been removed was investigated using an SEM. An epoxy replica method was employed to reveal the internal structure of etch pit. A 7 mm×7 mm square of electrograined aluminum plate was placed in a BEEM embedding capsule with the grained side up. The
capsule was subsequently filled with Spurr epoxy. Before cured at 60◦ C for 12 h, the capsule was evacuated within an oven using rotary pump for half an hour to ensure the epoxy completely fill in the whole space of etch pit. After peeling the capsule off, the epoxy-embedded aluminum plate was immersed in a 10% NaOH solution for 4 h to dissolve the aluminum substrate. The epoxy replica then duplicated the surface morphology of the electrograined aluminum plate. Cross-sectional TEM specimens were made from the as-grained aluminum plate using a combined mechanical grinding and ion-beam thinning method. Ion-beam thinning was carried out using Gatan dual-milling machine at a voltage of 5 keV. The time for ion milling was usually less than 2 h. TEM specimens were examined on a conventional JOEL200CXII TEM and a JOEL3010 analytical TEM. Along with detailed microstructure, particularly the pit morphology, energy-dispersive spectroscopy (EDS) was used for the composition analysis of etch film.
3. Results and discussion 3.1. Surface morphology of electrograined aluminum plates Fig. 1 shows a series of SEM micrographs illustrating the surface morphology change of aluminum plates with graining time. The as-rolled aluminum plate exhibits a relatively smooth surface with characteristic rolling lines, which are still discernible after 30 s of electrograining (arrow in Fig. 1(a)). Numerous pits form after 30 s of electrograining. Three types of pit were observed: the fine pit (marked as F in Fig. 1(a) and (b)), hemispherical pit (marked as H in Fig. 1(a) and (b)), and worm-like pit (marked as W in Fig. 1(a) and (b)). Most of the surface is dotted with fine pits. Several relatively large hemispherical and worm-like pits are also noted. The size of fine pits is usually far less than 1 m. The typical size of hemispherical pits is approximately 2 m. The typical length of worm-like pits is about 5 m. The orientation of worm-like pits is random with respect to the rolling direction. As the graining sequence proceeds to 60 s, most of the aluminum surface is dotted with pits. Growth and coalescence of fine pits result in the formation of more hemispherical pits. Meanwhile, the existing relatively large hemispherical pits show little growth. In addition, the population density and size of worm-like pits remains constant as electrograining time increases from 30 to 60 s. The original rolling lines are no longer visible at this stage of electrograining since most of the aluminum surface is dotted with pits. With further electrograining, more hemispherical pits form and the areas dotted with fine pits decrease. Concurrently, the worm-like pits merge with the growing hemispherical pits and presumably disappear (arrow in Fig. 1(c)). After 120 s of electrograining, most of the aluminum surface is dotted with relatively large hemispherical pits (Fig. 1(d)). The remaining fine pits always reside along some of the boundaries between hemispherical pits.
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Fig. 1. Series of SEM micrographs showing surface morphology change with electrograining time of (a) 30, (b) 60, (c) 90, (d) 120, (e) 180, (f) 240 s.
Note that at this stage of electrograining, the size distribution of pits is uniform, since more than 90% of the surface is dotted with hemispherical pits. With further electrograining (Fig. 1(e), 180 s of electrograining), the hemispherical pits grow laterally as they merge with the surrounding hemispherical pits. The average size of hemispherical pits is approximately 5 m. Meanwhile, the hemispherical pits propagate into the aluminum substrate via the formation of new pits on the walls of existing hemispherical pits. Consequently, the depth of hemispherical pits exhibits a more pronounced increase as electrograining time exceeds 180 s. Large and deep hemispherical pits form after 300 s of electrograining (Fig. 1(f)). Laevers et al. [7] investigated the influence of manganese on the AC electrolytic graining of aluminum. It is demonstrated that the degree of uniformity of the final pit morphology increases with decreasing amount of manganese present
in solid solution. For example, a uniformly pitted and convoluted surface is obtained for the 99.99% purity aluminum and the 1 wt.% manganese alloyed aluminum plate with most of manganese precipitated in intermetallic particles. Conversely, for 1 wt.% of manganese alloyed aluminum plate with most of the manganese (0.9 wt.%) present in solid solution, a lateral and unidirectional growth of hemispherical pits occurs, creating worm-like pits. The worm-like pits observed in the present work may arise from the alloying elements present in solid solution, such as iron, silicon and titanium for the AA1050 aluminum plate. The coexistence of hemispherical pits and worm-like pits suggests that the total amount of solid solution elements in the AA1050 aluminum plates is not enough to induce the prevalent formation of worm-like pits since the total amount of alloy elements present in AA1050 aluminum plate is approximately 0.5 wt.%.
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Fig. 2. Series of SEM micrographs showing internal structural change of etch pit with electrograining time of (a) 30, (b) 60, (c) 90, (d) 120, (e) 180, (f) 240 s.
3.2. Internal structure of etch pit Fig. 2 illustrates a series of SEM micrographs showing the internal structure of the electrograined surfaces. Similarly, the fine pit (marked as F in Fig. 2(a) and (b)), hemispherical pit (marked as H in Fig. 2(a) and (b)) and worm-like pit (marked as W in Fig. 2(a)) are the typical features on the aluminum surface after 30 s of electrograining. After 60 s of graining, most of the surface is dotted with etch pits (Fig. 2(b)). The walls of hemispherical pits are also dotted with many fine pits (arrow in Fig. 2(b)). As electrograining proceeds, growth and coalescence of fine pits lead to the formation of more hemispherical pits. Meanwhile, no apparent growth of the existing relatively large hemispherical pits was observed. The disappearance of worm-like pits arises from their merge with the hemispherical pits (arrow
in Fig. 2(c), 90 s of electrograining). Most of the aluminum surface is dotted with hemispherical pits after 120 s of graining (Fig. 2(d)). Growth and coalescence of fine pits on the walls of hemispherical pit form new hemispherical pits (arrow in Fig. 2(d)) inside the existing hemispherical pit. This is the typical process that the existing hemispherical pit propagates into the aluminum substrate. Meanwhile, lateral coalescence of hemispherical pits occurs progressively as electrograining proceeds, leading to the formation of the larger and deeper hemispherical pits (Fig. 2(e) and (f)). 3.3. Cross-sectional TEM microstructural characterization Cross-sectional TEM reveals more detailed morphology of various pits. Fig. 3 shows the morphology of a worm-like pit that is generally recognized by its longer length as com-
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Fig. 3. Cross-sectional TEM micrograph showing the microstructure of a worm-like pit (electrograined for 60 s).
Fig. 4. Cross-sectional TEM micrograph showing the microstructure of a hemispherical pit (electrograined for 90 s).
pared to the hemispherical pit and fine pit. For example, the typical length of the worm-like pits is approximately 5 m after 60 s of electrograining. Conversely, the size of relatively large hemispherical pits is approximately 2 m. It is evident that the worm-like pit is rather shallow in base, i.e. the pit is wider than its depth. The surface of worm-like pit is dotted with fine pits (arrow in Fig. 3), which are also
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Fig. 5. Cross-sectional TEM micrograph showing the microstructure of area dotted with fine pit and hemispherical pit (electrograined for 90 s).
hemispherical when resolved using TEM. The large pit on the samples electrograined more than 90 s is characterized as the hemispherical pit since the worm-like pits formed during the early stage of electrograining presumably disappear after 90 s of electrograining. Fig. 4 illustrates such an example. It is noted that the shape of hemispherical pit observed by SEM is not exactly hemispherical as shown by cross-sectional TEM. Besides, instead of curved surface, flat wall is observed associated with the hemispherical pit. Fig. 5 shows the morphology of the area dotted with fine pit (arrow) and hemispherical pit (double arrows) which also has a shallow base. Although the fine pits are close to hemisphere, their depths are smaller than those of hemispherical pits. Fig. 6 shows a large hemispherical pit on the aluminum surface after 120 s of electrograining. Again, the base of hemispherical pit is quite shallow, indicating that the lateral growth of hemispherical pit prevails its propagation into the aluminum substrate. Moreover, the hemispherical pit is across several subgrains. The structure of subgrain shows little effect on the morphology of the hemispherical pit. Conversely, the subgrain boundary has been identified as the pit nucleation site during the beginning of AC electrograining [9]. Electrochemical etching of aluminum in hydrochloric acid results in cubic pits [16,17]. Terryn and co-workers [6,7,11,13,14] concluded that the cubic pits are the basic
Fig. 6. Cross-sectional TEM micrograph showing the microstructure of a large hemispherical pit (electrograined for 120 s).
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Fig. 7. Cross-sectional TEM micrograph showing the etch pit structure of the AA1050 aluminum electrograined at 10 Hz.
building elements for pitted and convoluted surface of aluminum electrograined in hydrochloric acid. Characteristic size of the cubic pits decreases with increasing frequency of AC current. For electrograining frequency of 1 and 10 Hz, the characteristic size of cubic pit is about 1–2 and 0.3 m, respectively. The fine pits observed in the present work using cross-sectional TEM have a hemispherical shape. We also observed the cubic pits (arrow in Fig. 7) of characteristic size of 0.1 m decorated on the surface of the AA1050 aluminum plate, which is electrograined with the same graining conditions used for this study, except that 10 Hz electrograining frequency is adopted. Evidently, the size of basic building element is quite small when 50 Hz frequency is used, since the walls of fine pits are rather smooth. Terryn et al. [6,11] confirmed that the walls of hemispherical pits become smooth as the size of cubic pits decreases with increasing AC frequency. The formation of cubic pits indicates that the attack of aluminum in hydrochloric acid is along preferred crystallographic directions. The walls of hemispherical pits are flat instead of curved, suggesting that the pits develop along preferred crystallographic directions although the fine cubic pits cannot be resolved. The pits tend to grow laterally instead of propagation into the aluminum substrate since the base of hemispherical pit is shallow and flat. This is consistent with the observations made by Terryn and co-workers [6,7,11,13,14]. During AC etching, etch products always form to cover on top of grained surface. Fig. 8 shows a thin etch film on top of the electrograined surface. The etch film contains many fine pores (arrow in Fig. 8(a)), implying that the etch film is a porous structure. EDS reveals that the etch film is composed of aluminum and oxygen. This is consistent with the previ-
Fig. 8. (a) Cross-section TEM micrograph showing the microstructure of etch film on the as-grained aluminum surface and (b) the selected area diffraction pattern.
ous results illustrating that etch film is mainly composed of aluminum hydroxide [12,15]. Selected area diffraction pattern exhibits amorphous halo from the etch film, indicating that the etch film is amorphous (Fig. 8(b)). The diffraction spots (arrow in Fig. 8(b)) are from aluminum substrate. It is also noted that the amorphous etch film tends to crystallize when irradiated by 200 keV electrons. Therefore, instead of diffuse amorphous halo, the selected area diffraction pattern recorded shows faint amorphous halo. 3.4. Surface properties of electrograined aluminum plate Fig. 9 illustrates the dependence of Ra and Rmax on electrograining time. Ra and Rmax of electrograined aluminum plates increase progressively with electrograining time up to 300 s of graining. The lateral size of hemispherical pits as measured on SEM micrographs is larger than the corresponding Ra . This is consistent with the shallow-based hemispherical pits observed using cross-sectional TEM. The relative increase factor of aluminum plates increases abruptly during the first 30 s of electrograining treatment, which tends to roughen the aluminum surface (Fig. 10). The relative increase factor reaches a maximum after 90 s of electrograining, followed by a decrease after 120 s of electrograining, when most of the aluminum surface is dotted with the hemispherical pits. Evidently, the decrease in effective surface area arises from the disappearance of fine pits on the original aluminum surface. An increase in the relative increase factor is, however, noted when graining time increases from
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Fig. 9. Dependence of Ra and Rmax on the electrograining time.
Fig. 10. Dependence of peak count and relative increase factor on the electrograining time.
240 to 300 s. The fine pits decorated on the walls of deeper hemispherical pits are thought to bring about the increase of effective surface area of grained surface. The peak count of grained aluminum surface increases with electrograining time up to 120 s, when most of the fine pits have merged to form hemispherical pits. Peak count, then, decreases with electrograining time as the hemispherical pits grow and coalesce each other. Peak count remains constant as graining time exceeds 180 s, implying that new hemispherical pits formed inside the existing hemispherical pit compensates
for the decrease in peak count due to the coalescence of existing hemispherical pits. 4. Conclusions Pit growth of the AA1050 aluminum plates electrograined in hydrochloric acid was studied, with focus on the evolution of etch pits as electrograining proceeds. The fine pit, hemispherical pit and worm-like pit develop on the aluminum surface after 30 s of electrograining. The hemispherical pits
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form upon the coalescence of fine pits. Lateral and unidirectional growth of hemispherical pits result in the formation of worm-like pits. The population density of hemispherical pits increases with electrograining time. After 120 s of electrograining, a uniformly pitted surface is achieved with most of the aluminum surface dotted with hemispherical pits. Conversely, the population density and size of worm-like pits show little change as electrograining time increases from 30 to 60 s. Coalescence of the worm-like pits with hemispherical pits result in the breakdown of worm-like pits, which is then presumably phased out after 120 s of electrograining. The worm-like pit developed during the early stage of electrograining shows little effect on the uniformity of pit distribution after 120 s of electrograining. With further electrograining, lateral growth and coalescence of hemispherical pits result in the formation of larger hemispherical pit. In addition, deeper hemispherical pits form as fine pits nucleated on the walls of existing hemispherical pits merge each other to form new hemispherical pits inside the existing hemispherical pits. The hemispherical pit is not exactly hemispherical when observed using TEM. The base of hemispherical pit is shallow and its walls are flat. TEM resolves that fine pits also have a shape close to hemisphere. The etch film on top of the grained surface is identified as porous amorphous aluminum hydroxide. The subgrain structure shows little effect on the growth of hemispherical pit. The surface properties of electrograined aluminum plate correlate well with the corresponding pit morphology. Both Ra and Rmax increase as electrograining proceeds. A maximum peak count is achieved when most of the aluminum surface is dotted uniformly with hemispherical pits. The relative increase factor reaches a maximum value right before the stage when most of the fine pits merge into hemispherical pits.
Acknowledgements This research was supported by National Science Council, Republic of China, under grant No. 882216E212001. The authors would like to thank China Steel, Aluminum Corporation, for providing materials for this study. Special thanks to Mr. S.H. Hsieh of China Steel Corporation for his invaluable discussion. Also, we like to thank Mr. I.T. Wang, China Steel Corporation, for his help on carrying out the electrograining experiments.
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